How do you make Lens usage decision?

I always get confused with lens usage while traveling.

I have portrait prime as well as the walk lens as you can see in my signature. Both the lens have eligibility with portrait snaps but dont understand when it is absolutely necessary to use prime one and not the other one. Also the hassle of changing lens is impacting the decision as well.

There is that gray area where both lens can be used. The question is where do you draw the line for each case?

use the prime lens in low light condition or when you need to blur the background

I most always use my 17-40 as a walkaround, but I prefer the 'wide, close in' look, vs. a more remote telephoto approach. The nice thing about the 17-40 is that on a 1.6x camera it goes smoothly from wide angle to 'normal' lens, so that on the long end you are nearly emulating a FF 50mm lens.

i agree with lucky8 that primes are usually for low light or if you want that nice bokeh. zooms make good walkarounds because of their inherent flexibility. the 24-105mm f4 L IS IMHO is the perfect walkaround lens!

I prefer zooms for general use and the 17-40, 24-70 & 24-105 are all good choices. You have three very good lenses but I would imagine the 17-40 is the best all around lens for 90% of travelling needs (but always take them all wherever you go)!

nanko wrote in post #2080030
Both the lens have eligibility with portrait snaps...

Well, yes and no. I feel 40mm is just too short for head and shoulder shots of adults (the prospective of nose, ears, et.), 50-65mm is best.